The present invention relates generally to the conveyance of sheet materials and, more particularly, to an adjustable rack for supporting a glazing sheet in a generally horizontal position.
The use of racks or product supporting carriers are widely used within glass fabrication facilities for the transport of glass sheets, such as are intended for use as glazing closures in vehicles and the like, for example. Very often, these sheets must be heated to elevated temperatures and one expedient process for accomplishing this is to support individual sheets on racks and advance them through a heating furnace. One problem encountered in the conveyance of heat-softened glass sheets supported on conventional racks is the marring or scratching of the glass surfaces caused by the relative movement between such glass surfaces and the supporting surfaces of the rack, especially when direction of movement of the advancing glass-laden racks is changed or when they are transferred between right angularly related conveyor sections. This causes the heat-softened sheet to shift on the rack, tending to mark or scratch the glass surface along the regions of contact with the supporting surface of the rack, such defective surfaces often failing to meet the stringent optical requirements necessary in automotive glazing closures.
Another problem encounted in the heating of glass sheets carried on conventional racks is the adverse influence of the latter in altering the temperature profile of the sheets. For example, the relatively large, heat conducting, glass contacting surfaces of these known racks tend to conduct heat away from the glass surfaces to form cold spots and impose severe stresses therein, rendering the glass vulnerable to venting and breakage. Additionally, the metallic framework of racks absorbs heat and, with the glass sheet disposed in close proximity to such framework as in conventional racks, absorbs some of the heat otherwise intended for those glass portions adjacent the framework during heating while retarding the rate of cooling of such adjacent glass portions wheen subsequently removed from the heating atmosphere or furnace. These temperature differentials induced in the glass also impose undesirable stresses in the sheets to further increase the possibilities of venting and breakage. Moreover, conventional glass carrying racks do not offer protection for the glass sheets against the thermal currents inherently present in furnaces and which also produce thermal variations within a glass sheet passing through these furnaces.
Furthermore, since it is possible to process on the same production line a variety of differently sized glass sheets, it would be desirable to provide an adjustable rack for accommodating a variety of glass sizes rather than utilizing a supply of differently sized racks requiring large storage areas and sometimes special handling equipment.